Edit: or do you mean that previously there was room for slackers as long as they didn't care about promotion, and the difference now is their jobs are at risk?
The best anecdote I ever heard from an SRE was "the VP was chagrined when I told them it didn't matter I was in a nerf war, because nothing was broken or needed to be fixed, but they still saw my point and let me continue" (I used to work in an SRE area that overlapped with an SVP's preferred meeting room so Susan Wojcicki was always getting pinged by a rogue nerf bullet).
As for slackers? Sure, when I joined around 2008 there were definitely folks who were slacking off. in fact they got rid of sabbatical because so many people were taking it then coming back and quitting.
Yes, I think they collected lots of mediocre employees through the aggressive hiring.
Also, some of them may be not productive by common metrics (say bugs closed, lines of code implemented), others are good at those metrics, but produce high quantity/low quality work, and make more damage than impact: systems are overengineered, buggy, etc. I think second type is likely makes companies suffer much more than first one, they are much harder to trace and fire.